touchpad_st: read event fifo in heatmap mode
touchpad_st: read event fifo in heatmap mode

Beacon and error events are still available in heatmap mode.

BRANCH=nocturne
BUG=None
TEST=manual on device
Signed-off-by: Wei-Han Chen <5413a119f383c3f4549c3ef659403f393bf06e9d@chromium.org>

Change-Id: I6ac2e20f62bedf7084edb567765fd5b5fe7ae800
Reviewed-on: https://chromium-review.googlesource.com/1258689
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
Tested-by: Wei-Han Chen <5413a119f383c3f4549c3ef659403f393bf06e9d@chromium.org>
Reviewed-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec